Last year, there was a split between state and federal appellate courts on the question of whether the mobile phones of people being arrested can be searched without a warrant. The California Supreme Court held that the Fourth Amendment did not require a warrant for searches of mobile phones incident to a lawful arrest. In another case, the 1st Circuit Court of Appeals disagreed. Today, the Supreme Court sided clearly and convincingly with the latter: "Our answer to the question of what police must do before searching a cell phone seized incident to an arrest is ...simple—get a warrant." This is a significant victory for the Fourth Amendment.

Warrantless searches are presumptively considered "unreasonable" under the Fourth Amendment, but there are some exceptions to this general rule. One of these excpetions pertains to arrests. Typically, the police are permitted to conduct a warrantless search of a suspect's person during an arrest. As I argued earlier this year, however, there is good reason for this exception not to be applied to mobile phones. Because of the nature of mobile phones, the issues involved are very different than the typical search incident to an arrest, and the general exception shouldn't apply.

Speaking for a nearly unanimous court, Chief Justice Roberts agreed with the 1st Circuit that mobile phones are different than a typical search incident to arrest and require a warrant to be searched. U.S. v. Robinson, the 1973 case that created the exception for warrantless searches incident to an arrest, involved a simple seach of a suspect's person, which revealed heroin capsules. But Robinson, says Roberts, should not apply to cases like this: "neither of it rationales has much force with respect to digital content on cell phones." First, the data stored on mobile phones does not pose a safety threat to officers. And second, warrantless searches are not required to prevent the destruction of evidence. (The police, as even the defendants conceded, can secure the phones will obtaining a warrant to prevent data from being erased.)

Crucial to Roberts's argument is the fact that mobile phones contain personal information that police would typically have had to search a home in order to uncover. "The term 'cell phone' is itself misleading shorthand," observes Roberts. "[M]any of these devices are in fact minicomputers that also happen to have the capacity to be used as a telephone. They could just as easily be called cameras, video players, rolodexes, calendars, tape recorders, libraries, diaries, albums, televisions, maps, or newspapers." Both the quantity and quality of information contained on mobile phones is relevant. Not only do mobile phones frequently contain large amounts of information, this information is often highly personal: pictures, contacts, internet search histories.

The nature of the information in cell phones is crucial to the Court's Fourth Amendment analysis. In the 1969 case Chimel v. California, the Court held that the police could not conduct a general search of a suspect's home (as opposed to a search of the immediate surrounding area) without a warrant. The nature of modern mobile phones makes this case more like Chimel than Robinson. "Indeed," notes Roberts, "a cell phone search would typically expose to the government far more than the most exhaustive search of a house and hence according to the Court the police are required to obtain a warrant." Based on existing precedent, therefore, it should be as clear as is possible with a new technology that searches of mobile phones require a warrant, even incident to an arrest.

The precedent was in fact clear enough that all nine members of a Court with a notably dubious record on Fourth Amendment rights agreed with the judgment. The only justice who didn't fully join the Roberts opinion was Justice Samuel Alito, who sides most consistently with the state in Fourth Amendment cases. In a concurrence, Alito suggested that while the police cannot violate the Fourth Amendment on their own, they might be able to do it with the collaboration of Congress or state legislatures. "I would reconsider the question presented here if either Congress or state legislatures, after assessing the legitimate needs of law enforcement and the privacy interests of cell phone owners, enact legislation that draws reasonable distinctions based on categories of information or perhaps other variables," Alito wrote. Fortunately, no other member of the Court endorsed this dangerous invitation.

The basic principles of the Fourth Amendment need to be appropriately applied to modern technologies. The Court did so with respect to mobile phones today, preventing the further dilution of Fourth Amendment rights.